1. Field of the Invention
The present invention relates to a vapor-phase growth system and more particularly, to a cold-wall operated vapor-phase growth system having a reactor in which the inner space is divided into a growth chamber and a heater chamber by a holder and a substrate or wafer held by the holder in the space.
2. Description of the Prior Art
A vapor-phase growth system or equipment with a heating mechanism is usually used for thin-film growth processes such as a Low-Pressure Chemical Vapor Deposition (LPCVD) and a Molecular-Beam Epitaxy (MBE), or a process of forming a Hemi-Spherical Grained Silicon (HSG-Si) layer.
The conventional vapor-phase growth system has a reactor in which a thin film is grown on a substrate or J wafer. If the wall temperature of the reactor becomes high, in other words, the entire chamber becomes hot, during a growth process, it is said that the system is operated "hot-wall". If the wall temperature of the chamber does not become high, in other words, only the wafer becomes hot, during a growth process, it is said that the system is operated "cold-wall".
Also, when the reactor is designed for placing only one wafer in the reactor, it is called a "single-wafer" reactor. When the reactor is designed for placing a plurality of wafers in the reactor at a time, it is called a "batch" reactor.
The characteristics and uniformity of a thin film grown on the wafer is affected by cleanliness in the inner space of the reactor and uniformity of the wafer temperature. Considering this fact, various configurations and materials have been researched and developed in order to obtain a suitable support for the wafer.
A conventional cold-wall operated vapor-phase growth system is illustrated in FIGS. 1 to 3. As shown in FIG. 1, this system contains a water-cooled reactor 812 made of stainless steel, a cylindrical wafer holder 804 fixed in the reactor 812, a heater 806 provided in the reactor 812, a gas nozzle 816 fixed to the reactor 812 and communicated with a reaction gas source, and two turbo molecular pumps 808 and 809 connected to the reactor 812.
A top part 803 of the holder 804 is formed by a level plate, which is called a susceptor. The susceptor 803 supports a semiconductor substrate or wafer 801 on which a thin film or films are grown.
The heater 806, which is placed under the susceptor 803, raises the temperature of the wafer 801. The gas nozzle 816 supplies the reaction gas to the reactor 812. The turbo molecular pumps 808 and 809 pump out the air existing in the reactor 812 to thereby reduce its pressure.
The inner space of the reactor 812 is divided into two parts by the cylindrical wafer holder 804 and the wafer 801, one of which serves as a growth chamber 802 in which a thin film or films are grown on the wafer 801 using the supplied reaction gas in a vapor phase, and the other serves as a heater chamber 807 including the heater 806 therein.
The susceptor 803 is made of a circular-ringed outer part 803a and a circular-ringed inner part 803b, which is a double structure. The outer and inner parts 803a and 803b are made of quartz, for the following reasons:
(a) Since high purity quartz can be produced, the inner space of the reactor 812, i.e., the growth and heater chambers 802 and 807, can be kept clean. PA1 (b) Quartz has a sufficiently high temperature resistance against the wafer temperature during the film growth process. PA1 (c) Quartz has a sufficiently low heat-absorption rate of radiated heat from the heater 806.
The double structure functions to absorb deformation of the susceptor 803 due to thermal expansion with a clearance between the outer and inner parts 803a and 803b.
To be seen from FIGS. 2 and 3, the inner part 803b has an approximately circular window at its center to facilitate the transmission of the radiated heat from the underlying heater 806 to the wafer 801. The inner diameter of the inner part 803b is slightly smaller than the diameter of the wafer 801. The wafer 801 is placed onto the inner part 803b to be approximately concentric therewith. When the wafer 801 is placed onto the inner part 803b, the outer peripheral area of the wafer 801 contacts the inner peripheral area of the inner part 803b, as clearly shown in FIG. 3. The contact areas of the wafer 801 and the inner part 803b are very narrow.
The outer diameter of the inner part 803b is considerably larger than the diameter of the wafer 801. Therefore, almost the entire surface of the wafer 801 is exposed to the growth chamber 802.
The inner part 803b has a flat 803c engaged with the orientation flat 801c of the wafer 801.
As shown in FIG. 3, the outer part 803a is thicker than the inner part 803b, and has a small lateral protrusion at its inner side. The inner part 803b is placed on the protrusion to be engaged with the outer part 803a, thereby forming the double-structured susceptor 803. Almost all of the surface of the outer part 803a is exposed to the growth chamber 802.
The inner diameter of the outer part 803a is approximately equal to the outer diameter of the inner part 803b.
Since the contact area of the wafer 801 is heated through the inner part 803b of the susceptor 803 by the heater 806, the amount of the radiated heat tends to decrease compared with the case of the remainder of the wafer 801. Accordingly, if the susceptor 803 absorbs the radiated heat at a high rate, there is a problem that the temperature of the wafer 801 decreases in its contact area with the part 803b occurs. However, in the conventional system of FIGS. 1 to 3, the temperature decrease of the wafer 801 can be ignored, because the susceptor 803 is made of quartz having a sufficiently low heat-absorption rate.
As described above, with the conventional vapor-phase growth system, the double-structured susceptor 803 supports the wafer 801 and as a result, high temperature uniformity within the entire wafer 801 can be obtained while keeping the cleanliness high in the inner space of the reactor 812. However, the following problem occurs.
When a silicon thin film is grown on the wafer 801 in the conventional system, polycrystalline silicon (i.e., polysilicon) tends to be deposited on the inner part 803b of the susceptor 803 in the vicinity of the wafer 801, because the elevated temperature in the vicinity is approximately equal to that of the wafer 801. This deposited polysilicon becomes thicker with each repetition of the growth process, and finally, it is separated from the inner part 803b. The separated, deposited polysilicon not only becomes a source of particles but also causes the following serious problem.
Specifically, as already described above, the inner space of the reactor 812 is divided into the growth chamber 802 and the heater chamber 807 by the wafer holder 804 and the wafer 801 held by the holder 804, and both the chambers 802 and 807 are separately pumped out by the turbo molecular pumps 808 and 809. Therefore, the reaction gas supplied to the growth chamber through the nozzle 816 does not enter the heater chamber 807.
However, if the deposited polysilicon film is separated from the inner part 803b of the susceptor 803, the particles generated from the polysilicon film are deposited between the wafer 801 and the underlying inner part 803b, resulting in a gap therebetween. Then, the reaction gas is able to enter the heater chamber 807 through the gap. Consequently, unwanted polysilicon tends to be deposited on the heater 806 in the heater chamber 807, thereby changing the heating performance or characteristic of the heater 806.
For example, when the heater 806 is made of a resistive carbon and a quartz film formed to cover the carbon, polysilicon is deposited onto the quartz film in great volume. Since polysilicon has a larger heat absorption rate than quartz, the amount of heat applied to the wafer 801 tends to decrease, which leads to temperature lowering of the wafer 801.
The polysilicon film deposited onto the heater 806 becomes thicker with each repetition of the growth process, thereby increasing the amount of the absorbed heat. As a result, the practical temperature of the wafer 801 decreases even if the same power is supplied to the heater 806. This results in a problem that an obtainable thickness of the grown film fluctuates during the repeated growth processes.
Additionally, when a corrosive gas such as chlorine (C1) gas is used as the reaction gas, the lifetime of the heater 806 itself shortens due to the presence of the reaction gas.